Understanding Magnetic Nanochain Interactions for Reconfigurable Soft Actuators

Reconfigurable soft actuators hold great promise for applications ranging from mobile electronics to medicine, but they require more in-depth studies to improve the extent and efficiency of their actuation. Magnetic soft actuators are attractive candidates for these applications due to the extent of their deformation in external magnetic fields and the speed of their response. However, relatively little attention has been devoted to the influence of the shape and concentration of the magnetic filler (typically in the form of microparticles or other microstructures) on the properties of the assembled actuator. Here, we study the importance of these parameters and observe their influence on the actuator’s properties, ranging from its magnetic to its mechanical characteristics. <br/>In particular, this study delves into the potential of magnetic nanochains to offer soft actuators both the benefits of magnetic responsiveness and the additional feature of reconfigurability due to their enhanced anisotropy. We synthesized iron-cobalt nanostructures including nanoparticles and self-assembled nanochains using a magnetic field-free assembly method. The nanostructures were investigated to understand their individual and collective magnetic behaviors. After synthesis, an external magnetic field was applied to align the nanostructures. Nanochains largely remained in their single-particle-wide form but organized into longer lines of chains separated by regular distances; the synthesized nanoparticles, on the other hand, formed multi-particle-wide elongated strands with the width of a few micrometers. In both cases, alignment of the nanostructures led to an augmentation of their collective magnetic properties compared to when they were randomly oriented. However, the nanochains demonstrated a more pronounced enhancement (2x increase) in magnetic remanence compared to the case of nanoparticles. To further investigate the magnetic behavior of the nanochains and their potential as a filler for soft magnetic actuators, their properties were systematically studied as a function of filler concentration, i.e., spatial density. It was discovered that there exists a threshold concentration where the interactions between nanochains transition from ferromagnetic coupling to antiferromagnetic coupling. This resulted in a trend of initially increasing and subsequently decreasing remanence and coercivity as the concentration was increased. The maximum remanent concentration for a collective magnetic nanochain system was achieved at a filler concentration of 6 vol%, which yielded an M_r/M_s ratio approaching 0.5.<br/>Furthermore, we successfully fabricated a reconfigurable magnetic composite film by incorporating the optimal concentration of magnetic nanochains into an elastomer matrix. A soft actuator was made with two separate magnetic panels. The panels could be either encoded with the same magnetization orientation or opposite magnetization orientation to achieve various actuation modes. Subsequent reprogramming could be achieved through the application of a magnetic field to one or both panels. This actuator exhibited shape-morphing behaviors in the form of S-shaped twisting or either lateral or vertical U-shaped bending, respectively, in response to the magnetic field and could be repeatedly reprogrammed. Thus, we demonstrated the construction of reconfigurable magnetic soft actuators capable of large, efficient deformation in small actuation fields (less than 400 Oe). This research emphasizes the potential of magnetic nanochains as effective magnetic fillers and determines the optimal concentration of this magnetic filler for the development of reconfigurable, highly-elastic actuators.